1. Field of the Invention
The present invention relates to communication systems. More specifically, the invention relates to a communication system using a code division multiple access air interface between a plurality of individual subscribers distributed within a cellular community and a plurality of small capacity base stations, some colocated per cell to increase operational economy in proportion to the number of subscribers.
2. Description of the Prior Art
Advanced cellular communication makes use of a state of the art technique known as code division multiplexing, or more commonly, as code divisional multiple access or CDMA. An example prior art communication system is shown in FIG. 1.
CDMA is a communication technique in which data is transmitted with a broadened band (spread spectrum) by modulating the data to be transmitted with a pseudo-noise signal. The data signal to be transmitted may have a bandwidth of only a few thousand Hertz distributed over a frequency band that may be several million Hertz wide. The communication channel is being used simultaneously by m independent subchannels. For each subchannel, all other subchannels appear as noise.
As shown, a single subchannel of a given bandwidth is mixed with a unique spreading code which repeats a predetermined pattern generated by a wide bandwidth, pseudonoise (pn) sequence generator. These unique user spreading codes are typically orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. The data signal is modulated with the pn sequence producing a digital spread spectrum signal. A carrier signal is then modulated with the digital spread spectrum signal establishing a forward-link and transmitted. A receiver demodulates the transmission extracting the digital spread spectrum signal. The transmitted data is reproduced after correlation with the matching pn sequence. When the spreading codes are orthogonal to one another, the received signal can be correlated with a particular user signal related to the particular spreading code such that only the desired user signal related to the particular spreading code is enhanced while the other signals for all other users are not enhanced. The same process is repeated to establish a reverse-link.
If a coherent modulation technique such as phase shift keying or PSK is used for a plurality of subscribers, whether stationary or mobile, a global pilot is continuously transmitted by the base station for synchronizing with the subscribers. The subscriber units are synchronizing with the base station at all times and use the pilot signal information to estimate channel phase and magnitude parameters. For the reverse-link, a common pilot signal is not feasible. Typically, only non-coherent detection techniques are suitable to establish reverse-link communications. For initial acquisition by the base station to establish a reverse-link, a subscriber transmits a random access packet over a predetermined random access channel (RACH).
Most prior art CDMA communications systems employed to date, whether communicating with fixed or mobile subscribers that include personal communication services (PCS), have been designed for immediate large scale traffic considerations. A communication system specification proposed by a service provider establishes a required number of base stations which determine the region of communication coverage. The specification geographically locates each cell and establishes a traffic capacity that determines the number of anticipated subscribers per cell including fixed and mobile. The maximum capacity of communication traffic in each cell is typically fixed by this design.
Prior art CDMA communication systems have been designed and sized to immediately handle many simultaneous communications and are therefore costly start-up installations for the service provider. These systems have not addressed the need for a flexible base station architecture that permits a cost effective, small scale initial installation that can accommodate future subscriber growth.
Accordingly, the object of the present invention is to decrease the initial installation cost of a CDMA communication system while allowing future expansion when the need arises.